1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to an apparatus and method for accurately locating a UE (User Equipment) in the shadow area of GPS (Global Positioning System) satellite signals.
2. Description of the Related Art
With the ever-growing proliferation of personal, portable devices, one of the clear tendencies in modern society is to encourage and support provision of a positioning service for emergency services such as E911, (Emergency Call) service. Particularly, the United States seeks to deploy a nationwide, seamless communications infrastructure for the E911, service based on position information of UEs. It is provided by FCC (Federal Communication Committee) that the UE; positioning service should have an error of ±125M. With this relatively accurate UE positioning, it is possible to further provide other position information-related services.
The growing demands for the UE positioning service increases the importance of more accurate UE positioning. In general, a UE is located using Cell ID, AOA (Angle of Arrival), TOA (Time of Arrival), TDOA (Time Difference of Arrival), and at least three GPS (Global Positioning System) satellites.
In the Cell-ID method, a Node B, taking charge of cells and having information about its own accurate position, locates UEs within each cell based on its position. Since the positions of the UEs are determined on a cell by cell basis, they may be inaccurate. The AOA method is viable only when the angles between the Node B and the UEs are known through an intelligent antenna such as a smart antenna. The Node B can accurately locate the UEs based on the angles in transmitting data. Without the angles, therefore, it is impossible to accurately locate the UEs.
The TOA method locates a UE using the difference between a signal transmission time from the Node B and a signal arrival time at the UE. In the TDOA method, at least three Node Bs determine the position of a UE by triangulation of their transmitted signals to the UE. A distinctive shortcoming of the TDOA method is that it is impossible to accurately locate the UE in a radio channel environment having densely arranged obstacles like tall buildings, and thus many erroneous positioning factors like signal refraction and diffraction are present. The UE can also be located using GPS satellite signals. However, the GPS-based method is not feasible in an area where GPS satellite signals are not received such as a shadow area (e.g., subway).
Existing mobile communication systems can locate UEs in the above UE positioning methods either alone or in combination. It is noted that the GPS-based method is a more widespread used method because the use of GPS satellite signals provides an accurate absolute position of a UE. As described above, however, the GPS-based method cannot be carried out in the shadow areas.